I really like Renard too, and I think the show would be a lot poorer without him. It's out of character for him not to have warned Nick about Adelind, and another example of the writers dumbing down someone to further a plot device that would otherwise make no sense (a view which seems valid in the light of the sneak peeks from the finale
). He doesn't trust Adelind in the slightest, but his attitude is also coloured by some guilt over the babynapping and, I believe, an instinct to keep her where he can see her, so to speak. Of course, if Nick had spoken to him about the sudden influx of Grimms he might have reciprocated - we seem to be getting back to a previous level of non-communication of essential data.